Plasma confinement apparatus

ABSTRACT

PLASMA IS PRINCIPALLY CONFINED IN A TOROIDAL CONFIGURATION BY A STATIC MAGNETIC FIELD. FOR STABILISING THE PLASMA, A RADIOFREQUENCY ELECTROMAGNETIC FIELD MOVING FASTER THAN THE IONS IN THE PLASMA AND OF FREQUENCY IN THE REGION OF 1 MEGAHERTZ IS APPLIED. FOWER FOR GENERATING THE RADIOFREQUENCY FIELD IS COUPLED INTO AN ENDLESS PATH FOR CONDUCTING ELECTROMAGNETIC POWER, THE PATH HAVING A DISTRIBUTED INDUCTANCE AND CAPACITANCE SUCH THAT IT IS A CIRCULT WHICH RESONATES AT THE FREQUENCY OF THE ELECTROMAGNETIC FIELD.

y 4, 1972 c. J. H. WATSON 3,674,634

PLASMA CONFINEMENT APPARATUS Filed Jan. 21, 1969 0. c. VOLTAGE F7 SUPPLYIIIIIIIIIIIII I III I II I II I III II I I I I I I I I I I I I III IIIIIIIIIIIIIIIIIIIIIIIII l I r nite 3,674,634 Patented July 4, 19723,674,634 PLASMA CONFINEMENT APPARATUS Christopher John Hamilton Watson,Merton College, Oxford, England, assignor to United Kingdom AtomicEnergy Authority, London, England Filed Jan. 21, 1969, Ser. No. 792,553Claims priority, application Great Britain, May 22, 1968, 24,547 68 Int.Cl. G21b 1/00 US. Cl. 176-3 4 Claims ABSTRACT OF THE DISCLOSURE Plasmais principally confined in a toroidal configuration by a static magneticfield. For stabilising the plasma, a radiofrequency electromagneticfield moving faster than the ions in the plasma and of frequency in theregion of 1 megahertz is applied. Power for generating theradiofi'equency field is coupled into an endless path for conductingelectromagnetic power, the path having a distributed inductance andcapacitance such that it is a circuit which resonates at the frequencyof the electromagnetic field.

Cross reference is made to British patent specification No. 830,252 towhich United States Pat. specification No. 3,054,742 corresponds.

BACKGROUND OF THE INVENTION The invention relates to plasma confinementapparatus.

For the generation of thermonuclear power, for example, it is necessaryto device means for confining a hot plasma of thermonuclear fuel withoutthe plasma coming into contact with material boundaries. For theattainment of economic thermonuclear power it is also necessary, interalia, that the means for confining the hot plasma should not consumemore power than can be secured from thermonuclear reactions in thecontained plasma.

SUMMARY OF THE INVENTION The invention provides a plasma confinementapparatus comprising a vessel for containing a gas at low pressure,means for forming in or introducing into the vessel a plasma, means forproducing a static magnetic field te'nding to confine the plasma, meansfor producing a raido frequency electromagnetic field which movesrelatively to the plasma faster than the ions in the plasma are moving,the frequency of the electromagnetic field being in the region of lmegahertz, the said means for producing the radio frequencyelectromagnetic field including an electrical conductor forming anendless path for conducting electrical or electromagnetic power, whichpath has a distributed inductance and capacitance such that it is acircuit which resonates at the frequency of the electromagnetic field.

With a radio frequency electromagnetic field of fre quency in the regionof 1 megahertz, conductors used to localise the field may comprisesubdivided electrically conducting wires within which volume currentsflow.

Preferably the vessel is toroidal.

Preferably the electrical conductor forming the endless conducting pathcomprises a closed toroidal helix encompassing the vessel. Preferablythe electrical conductor is formed by forming the vessel with anelectrically insulating surface, depositing a thin layer of metal on thesurface and cutting a groove or grooves through the metal layer, thegroove or grooves following the required direction of the conductingpath.

DESCRIPTION OF PREFERRED EMBODIMENT A specific construction of apparatusembodying the invention will now be described by way of example and withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic illustration of part of the apparatus, and

FIG. 2 is a diagrammatic part sectional view of the apparatus.

In this example the plasma, illustrated at 11, is contained within atorus 12. The construction of such an apparatus and means for forming aplasma within the torus are described, for example, in US. Patent No.3,054,742 the US. counterpart of British patent specification No.830,252. Means for generating a static magnetic field has been showndiagramatically as winding 18 fed by a conventional D.C. supply 19.

Economic considerations can enable one to define a minimal economicthermonuclear plasma and the following approximate parameters for such aminimal economic theronuclear plasma have been deduced:

Surface thermonuclear energy flux P 250 watt/cm. Thermonuclear powerdensity P 1 watt/cm. Density n.: 5.3 10 ions/cm.

Temperature T: 19.5 kev.

Pressure p: 3.3 atmospheres Thermal energy density 3nT: 0.5 joules/cm.

Volume V: 10 cm.

Thermal Energy 62 5.10 joules Required energy confinement time: 1 secondVacuum vessel radius R: 18 metres These figures assume, for ease of theillustrative calculation, that the plasma is spherical with a radius rwhich is three quarters of the radius R of the surrounding vessel.

Confinement of such a plasma by radiation pressure alone would requirean electric field strength of approximately 2.7x l0 volts/cm. Fields ofthis strength might be obtained, although this has not yet beenachieved.

However, if this electromagnet field were localised around the plasma byenclosing it within a resonant cavity, the dissipation of power in thecavity walls would exceed the thermonuclear power generated by theplasma in all practical cases.

These prohibitive RF losses are a consequence of the skin effect, and itis clear that totally diiferent considerations apply once the frequencybecomes so low that one can subdivide the conductors used to localisethe RF field into thin wires, within which volume currents flow. Thisapproach becomes possible around 1 mc./s., and the factors which affectRF losses in this Waveband have to be considered. Since the skin depthin copper is still only around 0.1 mm. rigorous percautions have to betaken in order to approach the DC. level of ohmic losses. In view of thenecessarily heavy losses in the plasma at present laboratorytemperatures, there has been little motivation to reduce the ohmiclosses in the conductors. In consequence, existing circuits seldom havea quality factor Q exceeding 100. Whilst, on the scale of a smalllaboratory experiemnt is probable that the ohmic losses would remaindominant even if one reduced them to the DC. level, on the scale of areactor, however, radiation losses can become much more serious. A thirdsource of loss is the power dissipated by currents induced in imperfectnearby conductors or dielectrics.

The importance of minimising all of these loss processes can beemphasised by considering the RF power required tho confine a minimalthermonuclear plasma by means of a 1 mc./s. circuit of Q=l00. In theapparatus the volume occupied by the confining field is of the sameorder as the volume of the plasma, and the pressure balance conditionrequires their energy densities to be com- 3 parable. This requires astored electromagnetic energy e of order 5 joules, and the RF powerdissipated is therefore sou/Q2310 watts.

It is believed to be unlikely that this situation could be improved bythe required factor (10 by increasing Q alone: thus the low frequencyconfinement of the present apparatus has to operate in conjunction witha static magnetic field. A further factor to be taken into considerationis that proposals to make use of an RF magnetic field to confine theplasma involve the use of a condenser bank to store the (necessarilyequal) electrical energy in the circuit. At current prices, the cost ofa long life condenser bank capable of storing 5 10 joules is of order 210 which gives a cost of 400/kw.(e) of output. This figure isunacceptable by nearly two orders of magnitude. Thus, the low frequencyRF confinement system of this example has to meet the followingrequirements (i) the electrical energy has to be localised in the sameregion of space as the magnetic energy and not stored expensivelyelsewhere; (ii) a large fraction of the confinement has to be effectedby a static magnetic field, and (iii) the Q of the circuit has to beraised by many orders of magnitude above those achieved in existingradiofrequency confinement systems.

To meet the first requirement, the circuit must be designed so thatretardation elfects are important and hence, since the vacuum wavelengthat 1 mc./s. is much larger than reactor dimensions, it must take theform of a slow wave structure, wrapped around the plasma. The secondrequirement implies that the ion cyclotron frequency (2 in the staticmagnetic field would inevitably exceed 1 mc./s. This creates a geometricproblem, since RF fields can freely propagate through a plasma alongmagnetic lines of force at frequencies w below 9 and consequently exertno pressure on it. This geometric problem is solved in this example byemploying a toroidal magnetic field topology. The third requirementinvolves special precautions to minimise both ohmic and radiative loss.Considering first radiative loss, it can be shown that when the numberof terms N in the normal multipole expansion which is required torepresent the radiation is large, and when the overall radius a of theantenna is small compared with the vacuum wavelength, very highradiation Qs can be obtained. For example, for Wa/ =0.15, if

Thus, structures in which the RF currents have simple dipole orquadrupole representations have to be avoided. For meeting thisrequirement the configuration of the conductor for localising the RFfield in the apparatus of this example comprises a closed toroidal helixelectrical conductor represented diagrammatically in FIG. 1 at 13. Thisis a slow wave structure which permits RF power at any time of adiscrete set of resonant frequencies to circulate repeatedly around thetoroidal helix, until the power is dissipated by ohmic and/or radiationlosses.

It can be shown that, under given geometrical conditions, there arerelatively non-radiating resonant frequency modes in the toroidal helixin which the electromagnetic field is localised near the helix and, fora torus of minor radius approximately 1 meter, there exist such resonantmodes at a minimum frequency of the order of l mc./s.

The field structure at these frequencies is ideally suited to plasmaconfinement, since both the electric and magnetic fields have minima onthe circular axis of the torus and form a time-averaged minimum well inthe electric and magnetic fields. It is believed that the toroidal helixarrangement of this example may have a Q as high as 10 at a frequency of1 mc./s.

On this basis, the RF system of this example combined with (for example)a simple toroidal l/R static magnetic field to provide the mainconfining force, can lead to a reactor having the following approximateparameters. It

is assumed, as is optimum, that the surface flux is around the maximumpermissiblesay 1000 watts/cm. and that the torus has an aspect ratiowhich is as large as is compatible with a reasonable minor helixradius-say 1 metre:

RF power required P =5 10 where P p is the ratio of the RF pressure tothe plasma pressure and RF V is the ratio of the volume occupied by theRF field to that of the plasma. Estimates indicate a value of P -L4 10watts and the required radio frequency pressure is around 1.6atmospheres.

As mentioned above, it is important to reduce ohmic loss in conductor 13as far as possible. One technique for this is illustrated in FIG. 2. Thetorus 12 comprises an electrically insulating shell 14, on top of whichis deposited a layer of metal 14a of the order of 1 millimetre thick. Ahelical groove 15 is cut through the metal layer to leave a helicalconductor 13a.

A technique for coupling RF power into the closed helical conductor 13is illustrated in FIG. 1. A separate conductor 17, insulated from thehelical conductor 13, is wound around adjacent a few turns of thehelical conductor 13, thus providing a close electromagnetic couplinginto conductor 13 for RF power fed into conductor 17.

It will be appreciated that appropriate modification of part of thegroove cutting described with reference to FIG. 2 may be employed tointroduce the conductor 17 as part of the thin metal layer.

The invention is not restricted to the details of the foregoing example.For instance, other configurations of containment vessel and closedconductor may be devised within the requirements defined. One such otherconfiguration envisaged for the conductor is that of a generalisedtennis ball scam in which the closed loops along which the waves areguided lie on the surface of a sphere and have a number of symmetricallyarranged lobes, giving rise to an approximately multipole fieldconfiguration Within. This arrangement has the advantage over thetoroidal helix of allowing a significantly more favourable surface tovolume ratio (and hence lower minimum plasma pressure). A normal tennisball seam would probably have an unacceptably low radiation Q, so a moreconvoluted configuration is required. A difficulty lies in the choice ofthe accompanying static magnetic field. A magnetic mirror field can beexcluded: it is necessary to envisage a topologically toroidal field,for example that created by a straight current-bearing conductor passingthrough the centre, or that of a magnetohydrodynamic Hill vortex.

It should be further appreciated that, in the foregoing description, itis envisaged that the torus 12 carrying the toroidal winding 13, 13awill, in practice, be supported within an outer torus through whichcooling fluids, etc., flow. The calculations of radiative loss from thetoroidal windings, assuming radiation in free space, are thus, ofcourse, not directly applicable to the practical situation. However, thesame considerations apply, except that one is concerned with loss ofpower to the outer torus, rather than with loss of power by radiationinto free space.

The foregoing example has, effectively, subdivided electricallyconducting wires for providing the endless toroidal conducting path. Itis envisaged that the toroidal conducting path may, for example, bealternatively provided by a toroidal waveguide.

I claim:

1. A plasma confinement apparatus comprising a vessel for containing agas at low pressure, means for producing a static magnetic field tendingto confine the plasma, means for producing a radio frequencyelectromagnetic field which moves relatively to the plasma faster thanthe ions in the plasma are moving, the frequency of the electromagneticfield being in the region of 1 megahertz, the said means for producingthe radio frequency electromagnetic field including an elongatedelectrical conductor wrapped around the plasma and forming an endlesspath for conducting electrical or electromagnetic power, which path hasa distributed inductance and capacitance such that it is a circuit whichresonates at the frequency of the electromagnetic field.

2. A plasma confinement apparatus as claimed in claim 1, in which thevessel is toroidal.

3. A plasma confinement apparatus as claimed in claim 1 or claim 2, inwhich the electrical conductor forming References Cited UNITED STATESPATENTS 3,015,618 1/1962 Stix 176-3 3,156,621 11/1964 Josephson 176-33,219,534 11/1965 Furth 176-3 REUBEN EPSTEIN, Primary Examiner U.S. Cl.X.R.

